1use crate::iter::plumbing::*;
2use crate::iter::*;
3
4use std::fmt;
5use std::marker::PhantomData;
6
7/// Creates a parallel iterator that produces nothing.
8///
9/// This admits no parallelism on its own, but it could be used for code that
10/// deals with generic parallel iterators.
11///
12/// # Examples
13///
14/// ```
15/// use rayon::prelude::*;
16/// use rayon::iter::empty;
17///
18/// let pi = (0..1234).into_par_iter()
19/// .chain(empty())
20/// .chain(1234..10_000);
21///
22/// assert_eq!(pi.count(), 10_000);
23/// ```
24pub fn empty<T: Send>() -> Empty<T> {
25 Empty {
26 marker: PhantomData,
27 }
28}
29
30/// Iterator adaptor for [the `empty()` function](fn.empty.html).
31pub struct Empty<T: Send> {
32 marker: PhantomData<T>,
33}
34
35impl<T: Send> Clone for Empty<T> {
36 fn clone(&self) -> Self {
37 empty()
38 }
39}
40
41impl<T: Send> fmt::Debug for Empty<T> {
42 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
43 f.pad("Empty")
44 }
45}
46
47impl<T: Send> ParallelIterator for Empty<T> {
48 type Item = T;
49
50 fn drive_unindexed<C>(self, consumer: C) -> C::Result
51 where
52 C: UnindexedConsumer<Self::Item>,
53 {
54 self.drive(consumer)
55 }
56
57 fn opt_len(&self) -> Option<usize> {
58 Some(0)
59 }
60}
61
62impl<T: Send> IndexedParallelIterator for Empty<T> {
63 fn drive<C>(self, consumer: C) -> C::Result
64 where
65 C: Consumer<Self::Item>,
66 {
67 consumer.into_folder().complete()
68 }
69
70 fn len(&self) -> usize {
71 0
72 }
73
74 fn with_producer<CB>(self, callback: CB) -> CB::Output
75 where
76 CB: ProducerCallback<Self::Item>,
77 {
78 callback.callback(producer:EmptyProducer(PhantomData))
79 }
80}
81
82/// Private empty producer
83struct EmptyProducer<T: Send>(PhantomData<T>);
84
85impl<T: Send> Producer for EmptyProducer<T> {
86 type Item = T;
87 type IntoIter = std::iter::Empty<T>;
88
89 fn into_iter(self) -> Self::IntoIter {
90 std::iter::empty()
91 }
92
93 fn split_at(self, index: usize) -> (Self, Self) {
94 debug_assert_eq!(index, 0);
95 (self, EmptyProducer(PhantomData))
96 }
97
98 fn fold_with<F>(self, folder: F) -> F
99 where
100 F: Folder<Self::Item>,
101 {
102 folder
103 }
104}
105